Okkonen Stephen R.

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Stephen R.

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  • Article
    Climate variability, oceanography, bowhead whale distribution, and Iñupiat subsistence whaling near Barrow, Alaska
    (Arctic Institute of North America, 2010-06) Ashjian, Carin J. ; Braund, Stephen R. ; Campbell, Robert G. ; George, John C. ; Kruse, Jack ; Maslowski, Wieslaw ; Moore, Sue E. ; Nicolson, Craig R. ; Okkonen, Stephen R. ; Sherr, Barry F. ; Sherr, Evelyn B. ; Spitz, Yvette H.
    The annual migration of bowhead whales (Balaena mysticetus) past Barrow, Alaska, has provided subsistence hunting to Iñupiat for centuries. Bowheads recurrently feed on aggregations of zooplankton prey near Barrow in autumn. The mechanisms that form these aggregations, and the associations between whales and oceanography, were investigated using field sampling, retrospective analysis, and traditional knowledge interviews. Oceanographic and aerial surveys were conducted near Barrow during August and September in 2005 and 2006. Multiple water masses were observed, and close coupling between water mass type and biological characteristics was noted. Short-term variability in hydrography was associated with changes in wind speed and direction that profoundly affected plankton taxonomic composition. Aggregations of ca. 50–100 bowhead whales were observed in early September of both years at locations consistent with traditional knowledge. Retrospective analyses of records for 1984–2004 also showed that annual aggregations of whales near Barrow were associated with wind speed and direction. Euphausiids and copepods appear to be upwelled onto the Beaufort Sea shelf during Eor SEwinds. A favorable feeding environment is produced when these plankton are retained and concentrated on the shelf by the prevailing westward Beaufort Sea shelf currents that converge with the Alaska Coastal Current flowing to the northeast along the eastern edge of Barrow Canyon.
  • Article
    Optical, structural and kinematic characteristics of freshwater plumes under landfast sea ice during the spring freshet in the Alaskan coastal Arctic
    (American Geophysical Union, 2021-11-25) Okkonen, Stephen R. ; Laney, Samuel R.
    Rivers deliver freshwater and entrained terrestrial materials into the coastal ocean from adjacent continental landmasses. In the coastal Arctic, a large fraction of terrestrially sourced dissolved and particulate organic carbon (DOC and POC) is delivered by snowpack meltwaters of the spring freshet, when many coastal ocean regions remain covered by landfast ice. Here we report on an array of moored sensors and telemetering ice buoys deployed in advance of the 2018 spring freshet in Stefansson Sound near Prudhoe Bay, Alaska. This instrumented array monitored temporal and spatial variations in hydrographic properties before and during the freshet, as well as optical properties that serve as proxies for DOC and POC contained in the freshet plumes. The temporal evolution of these optical signals occurred in five stages, each associated with characteristic water column structural and kinematic characteristics. Spatial differences among fluorescent dissolved organic matter (FDOM) and optical backscatter (OBS) signals across the ice buoy array, evident later during the freshet, allowed identification of plume waters sourced from the Kuparuk, Sagavanirktok, and Shaviovik drainage basins.
  • Article
    An autonomous buoy system for observing spring freshet plumes under landfast sea ice
    (Association for the Sciences of Limnology and Oceanography, 2021-11-30) Laney, Samuel R. ; Okkonen, Stephen R.
    An ice buoy system was developed to measure oceanographic properties of freshwater plumes that occur in Arctic coastal oceans under landfast sea ice during the spring freshet. By implanting such systems into sea ice weeks or months in advance of the freshet event, sensors can be located immediately underneath the sea ice layer in situ at depths that riverine freshwater will occupy later when the freshet arrives. This observing approach is modular, can accommodate a wide range of sensors, is designed intentionally for use in remote regions, and can be readily deployed in any nearshore region that can be accessed by snowmachine. The buoy system incorporates an integral floatation collar that allows it to continue sampling as the coastal ocean becomes progressively ice free in the months after the freshet event. Automated sampling and telemetry via a satellite data network provide near-real-time observations of the timing and character of under-ice freshet plumes. An assessment study was done with an array of these ice buoy systems, outfitted with basic hydrographic and optical sensors and deployed in advance of the 2018 and 2019 freshets in landfast sea ice near the mouths of three coastal rivers in Stefansson Sound, Alaska.
  • Article
    Intrusion of warm Bering/Chukchi waters onto the shelf in the western Beaufort Sea
    (American Geophysical Union, 2009-06-27) Okkonen, Stephen R. ; Ashjian, Carin J. ; Campbell, Robert G. ; Maslowski, Wieslaw ; Clement-Kinney, Jaclyn L. ; Potter, Rachel
    Wind-driven changes in the path of warm Bering/Chukchi waters carried by the Alaska Coastal Current (ACC) through Barrow Canyon during late summer are described from high-resolution hydrography, acoustic Doppler current profiler–measured currents, and satellite-measured sea surface temperature imagery acquired from mid-August to mid-September 2005–2007 near Barrow, Alaska. Numerical simulations are used to provide a multidecadal context for these observational data. Four generalized wind regimes and associated circulation states are identified. When winds are from the east or east-southeast, the ACC jet tends to be relatively strong and flows adjacent to the shelf break along the southern flank of Barrow Canyon. These easterly winds drive inner shelf currents northwestward along the Alaskan Beaufort coast where they oppose significant eastward intrusions of warm water from Barrow Canyon onto the shelf. Because these easterly winds promote sea level set down over the Beaufort shelf and upwelling along the Beaufort slope, the ACC jet necessarily becomes weaker, broader, and displaced seaward from the Beaufort shelf break upon exiting Barrow Canyon. Winds from the northeast promote separation of the ACC from the southern flank of Barrow Canyon and establish an up-canyon current along the southern flank that is fed in part by waters from the western Beaufort shelf. When winds are weak or from the southwest, warm Bering/Chukchi waters from Barrow Canyon intrude onto the western Beaufort shelf.
  • Article
    Ecological characteristics of core-use areas used by Bering–Chukchi–Beaufort (BCB) bowhead whales, 2006–2012
    (Elsevier, 2015-09-10) Citta, John J. ; Quakenbush, Lori T. ; Okkonen, Stephen R. ; Druckenmiller, Matthew L. ; Maslowski, Wieslaw ; Clement-Kinney, Jaclyn L. ; George, John C. ; Brower, Harry ; Small, Robert J. ; Ashjian, Carin J. ; Harwood, Lois A. ; Heide-Jørgensen, Mads Peter
    The Bering–Chukchi–Beaufort (BCB) population of bowhead whales (Balaena mysticetus) ranges across the seasonally ice-covered waters of the Bering, Chukchi, and Beaufort seas. We used locations from 54 bowhead whales, obtained by satellite telemetry between 2006 and 2012, to define areas of concentrated use, termed “core-use areas”. We identified six primary core-use areas and describe the timing of use and physical characteristics (oceanography, sea ice, and winds) associated with these areas. In spring, most whales migrated from wintering grounds in the Bering Sea to the Cape Bathurst polynya, Canada (Area 1), and spent the most time in the vicinity of the halocline at depths <75 m, which are within the euphotic zone, where calanoid copepods ascend following winter diapause. Peak use of the polynya occurred between 7 May and 5 July; whales generally left in July, when copepods are expected to descend to deeper depths. Between 12 July and 25 September, most tagged whales were located in shallow shelf waters adjacent to the Tuktoyaktuk Peninsula, Canada (Area 2), where wind-driven upwelling promotes the concentration of calanoid copepods. Between 22 August and 2 November, whales also congregated near Point Barrow, Alaska (Area 3), where east winds promote upwelling that moves zooplankton onto the Beaufort shelf, and subsequent relaxation of these winds promoted zooplankton aggregations. Between 27 October and 8 January, whales congregated along the northern shore of Chukotka, Russia (Area 4), where zooplankton likely concentrated along a coastal front between the southeastward-flowing Siberian Coastal Current and northward-flowing Bering Sea waters. The two remaining core-use areas occurred in the Bering Sea: Anadyr Strait (Area 5), where peak use occurred between 29 November and 20 April, and the Gulf of Anadyr (Area 6), where peak use occurred between 4 December and 1 April; both areas exhibited highly fractured sea ice. Whales near the Gulf of Anadyr spent almost half of their time at depths between 75 and 100 m, usually near the seafloor, where a subsurface front between cold Anadyr Water and warmer Bering Shelf Water presumably aggregates zooplankton. The amount of time whales spent near the seafloor in the Gulf of Anadyr, where copepods (in diapause) and, possibly, euphausiids are expected to aggregate provides strong evidence that bowhead whales are feeding in winter. The timing of bowhead spring migration corresponds with when zooplankton are expected to begin their spring ascent in April. The core-use areas we identified are also generally known from other studies to have high densities of whales and we are confident these areas represent the majority of important feeding areas during the study (2006–2012). Other feeding areas, that we did not detect, likely existed during the study and we expect core-use area boundaries to shift in response to changing hydrographic conditions.
  • Article
    Lingering Chukchi Sea sea ice and Chukchi Sea mean winds influence population age structure of euphausiids (krill) found in the bowhead whale feeding hotspot near Pt. Barrow, Alaska
    (Public Library of Science, 2021-07-12) Ashjian, Carin J. ; Okkonen, Stephen R. ; Campbell, Robert G. ; Alatalo, Philip
    Interannual variability in euphausiid (krill) abundance and population structure and associations of those measures with environmental drivers were investigated in an 11-year study conducted in late August–early September 2005–2015 in offshelf waters (bottom depth > 40 m) in Barrow Canyon and the Beaufort Sea just downstream of Distributed Biological Observatory site 5 (DBO5) near Pt. Barrow, Alaska. Statistically-significant positive correlations were observed among krill population structure (proportion of juveniles and adults), the volume of Late Season Melt Water (LMW), and late-spring Chukchi Sea sea ice extent. High proportions of juvenile and adult krill were seen in years with larger volumes of LMW and greater spring sea ice extents (2006, 2009, 2012–2014) while the converse, high proportions of furcilia, were seen in years with smaller volumes of LMW and lower spring sea ice extent (2005, 2007, 2010, 2011, 2015). These different life stage, sea ice and water mass regimes represent integrated advective responses to mean fall and/or spring Chukchi Sea winds, driven by prevailing atmospheric pressure distributions in the two sets of years. In years with high proportions of juveniles and adults, late-spring and preceding-fall winds were weak and variable while in years with high proportions of furcilia, late-spring and preceding-fall winds were strong, easterly and consistent. The interaction of krill life history with yearly differences in the northward transports of krill and water masses along with sea ice retreat determines the population structure of late-summer krill populations in the DBO5 region near Pt. Barrow. Years with higher proportions of mature krill may provide larger prey to the Pt. Barrow area bowhead whale prey hotspot. The characteristics of prey near Pt. Barrow is dependent on krill abundance and size, large-scale environmental forcing, and interannual variability in recruitment success of krill in the Bering Sea.
  • Article
    Optical insight into riverine influences on dissolved and particulate organic carbon in a coastal arctic lagoon system
    (American Geophysical Union, 2023-04-04) Catipovic, Luka ; Longnecker, Krista ; Okkonen, Stephen R. ; Koestner, Daniel ; Laney, Samuel R.
    Arctic coastal margins receive organic material input from rivers, melted sea ice, and coastal erosion, phenomena that are all undergoing changes related to global climate. The optical properties of coastal Arctic waters contain information on this organic material, and we examined three optical properties of seawater (absorption, backscatter, and fluorescence) for their relationships to variability in dissolved and particulate organic carbon (DOC and POC) in Stefansson Sound, Alaska, a coastal Arctic embayment. During open water periods in 2018 and 2019, DOC was inversely correlated with salinity (r2 = 0.97) and positively correlated with dissolved organic matter fluorescence (fDOM; r2 = 0.67). DOC showed strong correlation with the nonparticulate absorption coefficient at 440 nm (ag(440)) only in 2018 (r2 = 0.95). The vertical structure of fDOM in Stefansson Sound aligned with density profiles more strongly in 2018 than in 2019, and higher levels of fDOM, ag(440), and backscatter seen near the bottom in 2019 suggest wind‐driven mixing and/or bottom resuspension events. In both years, DOC correlated strongly with the spectral slope of the absorption coefficient between 412 and 550 nm (r2 = 0.70), and POC was well correlated with spectral backscattering at 470, 532, and 660 nm (r2 = 0.90, 0.71, and 0.59). These interannual differences in the spatial and vertical distributions of DOC and POC, and their respective correlations with optical proxies, likely reflect regional climatological factors such as precipitation over the adjacent watersheds, wind patterns, and residual sea ice in late summer.